  void cmd_del(istringstream& ss) {
    string sfd;
    if(!(ss>>sfd)) throw "syntax: del all|name-of-fd-domain";
    
    if(sfd=="all") {
      for(int i=0;i<(int)fd.size();i++) delete_fd(i);
    } else {
      int nfd=search_fd(sfd);
      if(nfd<0) throw "can not find fd domain";
      delete_fd(nfd);
    }
  }
